The present invention relates to high-chi block copolymers for forming interconnect structures by directed self-assembly, and more specifically to methods of forming contact holes and contact bars of small critical dimension (CD) and good uniformity.
A semiconductor device typically includes a network of circuits that are formed over a substrate. The device can consist of several layers of circuit wiring, with various interconnects being used to connect these layers to each other and any underlying transistors. Generally, the interconnects of the patterning layer can have the form of contact holes (vias) or contact bars. The contact holes and/or bars are transferred to an underlying layer and filled with a metal to form the interconnects that allow the various layers of circuitry to be in electrical communication with each other.
Methods of forming interconnects generally rely on a series of lithographic and etching steps to define the positions and dimensions of openings (e.g., holes, bars), which in turn define the positions and dimensions of the corresponding interconnects. To this end, photoresists and hard masks can be employed. However, the dimensions of features formed using conventional optical lithography techniques for volume manufacturing (e.g., 193 nm dry and immersion lithography) have reached the resolution limit of the lithographic tools. For example, the creation of vias with adequate CD uniformity at smaller pitch is one of the major challenges for future technology nodes. The International Technology Roadmap for Semiconductors (ITRS) requires an overall CD variation (i.e., 3 sigma (3σ) variation, where sigma is the standard deviation of the critical dimension) of less than 10% of the CD to ensure reasonable device performance. However, this is expected to be difficult for contact hole diameters less than 20 nm using conventional optical lithography, even with expensive and complicated double patterning processes, resolution enhancement technology (computational lithography), and severe layout design restrictions.
Block copolymers (BCPs) find many applications in solution, bulk and thin films. BCPs for directed self-assembly (DSA) applications comprise two or three polymer blocks that can phase-segregate into domains characterized by ordered nanoscopic arrays of spheres, cylinders, gyroids, and lamellae, which can have a feature size from about 5 nm to about 50 nm. When utilized with existing photolithographic techniques, thin-film self-assembly properties of BCPs potentially provide a unique approach to creating domain patterns having long range order and smaller dimensions than the patterning capabilities of conventional lithography.
One DSA technique is grapho-epitaxy, in which self-assembly of a BCP is guided by the topography and surface properties of the features of a lithographically pre-patterned substrate. In pre-patterns comprising holes and bars having sidewalls descending into a layer of a substrate, the sidewall surfaces can guide BCP self-assembly.
The ability of a BCP to phase-segregate depends on the Flory Huggins interaction parameter chi (χ). Poly(styrene)-block-poly(methyl methacrylate), abbreviated as PS-b-PMMA, is the most widely used block copolymer for DSA. However, the minimum pitch of PS-b-PMMA is limited to about 20 nm because of lower interaction and interaction parameter (χ) between the PS and PMMA blocks.
To achieve further feature miniaturization of interconnects, a block copolymer having a high interaction parameter between two blocks (higher chi) is desired, which self-assembles to form uniform cylindrical or lamellar domains of smaller dimension compared to the pre-pattern opening in which DSA takes place.